1. Field of the Invention
The invention relates to drill bits and methods of manufacturing drill bits, and is particularly applicable to drag type drill bits. More specifically, the present invention relates to drill bits formed by powder metallurgy wherein the cutter assemblies are brazed into pockets formed in the bit matrix.
2. Description of the Prior Art
The use of drag type drill bits in the drilling of wells, particularly oil and gas wells, is well known. The typical drag type drill bit serves to transfer the weight of the drill string to the bottom of the borehole. The protruding cutters on the bottom surface of the drill bit serve to cut into the formation as the drill bit rotates. In extremely hard formations, the cutters essentially crush or fracture the formation by means of the large compressive force applied to the formation. In such hard formations, the cutters may take the form of relatively small diamonds, e.g., 1/10 carat. In soft formations, the drag type drill bit essentially plows through the formation. The typical drill bit used in relatively soft formations may utilize large cutters, e.g., 2 karat.
Numerous considerations must be taken into account in the design of a drag type bit. As mentioned above, the cutter size is typically dictated by the hardness of the formation. Furthermore, the cutter may be chosen to have a specific profile, e.g., round or V-shaped. The concentration and placement of the diamonds along the face of the drill bit are critical to drill performance.
Careful attention must be given to the hydraulic characteristics of the bit since the drill bit must remain relatively cool and the cuttings must be promptly swept away from the drilling interface so that they are not reground. To this end, intricately designed waterways or fluid courses are provided on the face of the drill bit to direct the drilling mud across the cutting face to effect cooling and cleaning. Typical hydraulic designs provide for either radial flow or circumferential flow, or some combination thereof. The hydraulics of the bit are also affected by the "profile" of the bit, i.e., the radius of curvature at the face of the bit. Nozzles may be used to develop desired hydraulic conditions.
Commercial drag type drill bits presently in use are typically formed by powder metallurgy techniques wherein a graphite mold is made to the shape of the bit. Depressions are carefully located in the mold and natural diamond cutters are glued into place in the depressions. A tungsten carbide powder is placed into the mold and infiltrated in a furnace cycle with a copper alloy with a steel shank in place. The maximum temperature in the furnace cycle may be on the order of 2200.degree. F. The bit is allowed to drop out of the mold and is finished by welding to the steel shank an extension including the pin, and final machining.
Recent developments in the manufacture of drag type drill bits suggest that synthetic polycrystalline diamond drill blanks may be utilized as the cutters in such bits. These synthetic cutters have the unique advantage of being uniformly shaped, as opposed to the varying shapes of natural diamonds. However, present synthetic diamond drill blanks cannot be placed in the matrix prior to furnacing as can natural diamonds because the synthetic diamond cannot withstand temperatures on the order of 2200.degree. F. Synthetic polycrystalline diamonds in a disc form have been brazed directly onto the matrix of drag drill bits for use in soft formations. To date, the synthetic diamond discs which have been secured to drag type drill bits have been relatively large, e.g., 1/2 inch in diameter, because of limitations in the reliable attachment of the cutter to the drill bit matrix. Because the cutters are not located in a single plane, some form of positive fixturing must be used when attaching the cutters to the previously formed and furnaced drill bit head. It has been proposed to use dead weights and various camming arrangements to fixture synthetic diamond cutters during brazing operations. However, such fixturing techniques have proven extremely complex and unreliable. It also has been suggested to fixture the synthetic diamond cutters by the use of shims of a high expanding metal that expands during the heating of the brazing operation and is removable upon cooling. This approach has also proved disadvantageous.
Thus, it can be seen that there is an acute need for a drill bit having securely attached cutters, for example, synthetic polycrystalline diamond cutters, attached after furnacing of the bit head matrix. Improved attachment methods make possible the use of relatively small cutters for use in medium and hard formations.